The “solder paste” as referred to in the invention of the present application is a paste-like solder (typically also referred to as “solder cream” and “soldering paste”) obtained by mixing (kneading) a solder alloy powder and a flux. Tin-lead systems have been conventionally used as solder alloys, but a transition to lead-free solders (containing no lead) has been made in recent years with consideration for hazardousness of lead.
Heat resistance of individual electronic components in substrate mounted structures of electronic components has recently increased. However, as the demand for valve control such as slot valve or air intake valve control has been growing following the increased in functionality of vehicles, it is required that no cracks occur in soldered portions of a mounting substrate even in a 1000-cycle heat-shock test of −40° C. to 150° C. that is required for components actually used around the engine room of a vehicle, that is, in the vicinity of an engine. It is also necessary that lead-free solder alloy materials be used to reduce environmental load, and solder pastes are also needed that enable reflow mounting with solder alloy materials having high heat resistance.
Thus, Sn—Ag—Bi—In solder alloy materials have been suggested as lead-free solder alloy materials that are not only suitable for applications requiring high heat resistance, but also have good mechanical properties and can be used for substrate mounting (see, for example, Patent Document 1).
Mounted structures of electronic components in which solder joint reliability is increased and heat resistance is also increased by using Sn—Cu or Sn—Ag solder alloy materials and expensive substrates from a metal core or a ceramic material with high heat radiation ability as substrate materials, thereby increasing heat resistance of substrate mounting, have been suggested (see, for example, Patent Document 2).
It has also been suggested to include a carboxylic-acid-including viscosity increase inhibitor that is compatible with resin components with a low dissociation constant into a flux (see, for example, Patent Document 3) and include a dicarboxylic acid that is a ring-opened solid body at normal temperature (see, for example, Patent Document 4) in a solder paste to improve substrate mounting ability.